The objective of this project is to determine the optimal timing of surgery for intracranial aneurysms. Approximately 26,000 pateints have a subarachnoid hemorrhage (SAH) from aneurysm in the United States each year. Most are young, healthy individuals with good neurological function post-hemorrhage with the potential for returning to a vigorous and productive life. Many will develop devastating neurological deficits or perish from rebleeding or vasospasm; less than 50% of those patients who reach hospital will have a favorable outcome. There is considerable room for improvement. After SAH approximately 35% of patients will die from rebleeding. Surgery effectively prevents this complication. Timing of surgery is the most important question facing clinicians involved in the management of these cases. There is no uniformity amongst neurosurgeons as to the optimal timing. Most surgeons advocate waiting until the patient's neurological condition is near normal; others prefer to wait an arbitrary length of time even in neurologically intact patients; still others feel the patients should be operated upon as soon as convenient after the hemorrhage. These divergent views are supported in the literature only by anecdotal experiences; no definitive information is available for guidance. Obviously the longer the waiting period to surgery the greater will be the chances for rebleeding and death. However, it is generally thought that the lowest operative morbidity and mortality occurs in good-condition patients in whom surgery is delayed at least 2 weeks. In fact, no valid statistical analysis exists to prove that late surgery produces better results than early surgery with contemporary aids. In attempt to answer this perplexing question regarding timing of surgery, a prospective study at varying intervals after SAH will be performed. To obtain a sufficient number of cases in a reasonable time frame, an international cooperative study will be utilized.